1. Field of the Invention
The present invention is related to an objective lens device having a high numerical aperture, and more particularly, to an objective lens device for correcting chromatic aberration due to a change in wavelength of light and/or an increase in line width of the wavelength of light emitted from a light source while having a high numerical aperture to realize high-density light concentration, and to an optical pickup employing the same.
2. Description of the Related Art
Recording capacity of an optical recording/reproducing apparatus is determined by a size of a light spot formed on an optical disc by an objective lens. The size (S) of the light spot is proportional to a wavelength xcex and inversely proportional to a numerical aperture (NA) of the objective lens.
Thus, an optical pickup device for a next generation DVD (a so-called HD-DVD), (hereinafter, a high-density optical pickup device) which is presently being developed, may adopt a light source for emitting blue light and an objective lens having an NA greater than 0.6 to obtain a recording density higher than an information recording density obtained from a conventional CD or DVD based optical disc by reducing the size of the light spot formed on the optical disc.
Optical materials, such as for example glass and plastic, which are used as material of the objective lens exhibit a sharp change in refractive index in a range of a wavelength shorter than 650 nm, as shown in Table 1.
As can be seen from Table 1, where the wavelength changes about 1 nm, optical material exhibits a four times greater change in refractive index in a blue wavelength range, for example, 405 nm, than in a 650 nm wavelength which is used for an optical pickup device for a DVD,
The above sharp change in refractive index in the optical material with respect to the blue light is one of the major causes of deterioration in performance according to defocus in a recordable high-density optical recording/reproducing apparatus which records and reproduces repeatedly by using a blue wavelength light source.
That is, in the recordable optical recording/reproducing apparatus, optical power for recording and optical power for reproduction which are different from each other are used. A change in wavelength according to a change in optical output power during recording/reproduction is, for example, between 0.5-1 nm with respect to a blue light source. Usually, if output of a light source is increased, the wavelength of light emitted from the light source becomes longer. Thus, in the case of a high-density optical pickup device adopting a blue light source, a great amount of chromatic aberration, due to a change in wavelength in switching optical output between recording and reproduction in an objective lens designed with respect to a reference wavelength, is generated so that defocus is generated.
For example, as can be seen from FIGS. 1 through 3, an objective lens device having a 0.65 NA which is designed with respect to a 405 nm wavelength exhibits great wavefront aberration and defocus with respect to a tiny change of about 1 nm in wavelength. FIG. 1 is a graph showing light spot strength formed on an optical disc according to defocus due to a change in optical output power during recording/reproduction. FIGS. 2 and 3 are graphs showing amounts of wavefront aberration (optical path difference: OPD) and defocus of an objective lens device having a 0.65 NA according to a change of wavelength, respectively.
Although the defocus due to a change in wavelength is correctable by adjusting the objective lens device, since tracking a wavelength change by driving the objective lens device by an actuator takes a relatively long time, the quality of reproduction and recording signals deteriorates during the period of driving the objective lens. Defocus during an increase in output for recording causes a lack of recording optical power while defocus during a decrease in output for reproduction increases jitter.
That is, where the output of a light source is increased to record information on an optical disc, the wavelength of light emitted from the light source is lengthened to, for example, 406 nm, so that defocus is generated to a light spot formed on the optical disc. Thus, until the actuator tracks the defocus, recording is not performed properly. Where the output of a light source is decreased for reproduction, the wavelength of the light source is shortened to, for example, 405 nm. In this case, since the actuator is in the state of tracking suitable for the lengthened wavelength, defocus is generated again. Due to the defocus, jitter increases in a reproduction signal.
Also, where the light source is driven with a high frequency (HF) to reduce feed-back noise of a light source due to light returning to the light source from the optical disc, a line width of a wavelength of the light source increases to, for example, about 1 nm, and resulting chromatic aberration deteriorates a reproduction signal.
Thus, a rewritable high-density optical pickup device needs to have an optical system which restricts or compensates for generation of chromatic aberration even where the wavelength of the light emitted from the light source changes according to a change in recording and reproduction output.
A conventional objective lens device having two lenses having a function to correct chromatic aberration has been suggested in Japanese Patent Publication No. 10-123410. Referring to FIG. 4, the conventional objective lens device includes first and second lenses 1 and 4 formed of low dispersive glass having an Abbe number which is greater than 40. At least one surface of the first and second lenses 1 and 4 is formed to be aspherical, so that correction of chromatic aberration and a high NA are realized with respect to light having a 635 nm wavelength. The chromatic aberration is corrected by the first lens 1 provided between a disc 6 and the second lens 4 for concentrating light and the objective lens device has an NA greater than 0.7. A diaphragm 2 restricts a light input area.
However, since the conventional objective lens device is formed of two lenses in two groups and uses low dispersive glass having an Abbe number which is greater than 40, the correction of chromatic aberration and a high NA is realized with respect to light having a 635 nm wavelength. However, the conventional objective lens device does not realize the correction of chromatic aberration and a high NA with respect to blue light.
To solve the above problems, it is an object of the present invention to provide an objective lens device which corrects chromatic aberration with respect to a blue light and realizes a high numerical aperture, and an optical pickup employing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above and other objects of the invention, there is provided an objective lens device comprising three lenses including a lens having a negative power and formed of material having an Abbe number which is 45 or less in line d, wherein at least one surface of the three lenses is aspherical. Preferably, at least one of the three lenses has a positive power and the lens having the negative power is combined with a lens having a positive power to be a doublet so that a structure of three lenses in two groups is formed.
Preferably, the three lenses are a first lens having a positive power, a second lens having a negative power, and a third lens having a positive power, arranged sequentially from a direction from which light is input. Preferably, given that a focal length of the lens having the negative power is ƒn and an overall focal length of the objective lens device is ƒ, the objective lens device satisfies an equation of       -    2.4     less than       fn    f     less than       -          1.4      .      
To achieve the above and other objects of the invention, there is provided an optical pickup comprising a light source which generates and emits light, an objective lens device which focuses the light emitted from the light source into a light spot formed on a recording medium, an optical path changer, arranged on an optical path between the light source and the objective lens device, which changes a proceeding path of incident light, and a photodetector which receives light reflected by the recording medium, wherein the objective lens device comprises three lenses wherein one of the three lenses has a negative power and formed of material having an Abbe number which is 45 or less in line d, and at least one surface of surfaces of the three lenses is aspherical.
Preferably, the light source emits light having a wavelength in a range including 400 and 420 nm and the objective lens device has a numerical aperture which is 0.70 or greater.